1. Field of the Invention
The present invention relates to a method of manufacturing a surface acoustic wave device such as a surface acoustic wave filter, a surface acoustic wave resonator, or the like, and more particularly, to a method of manufacturing a surface acoustic wave device in which a step of adjusting the frequency thereof is improved.
2. Description of the Related Art
A surface acoustic wave device is used to define various components such as a filter, a resonator, and the like. In manufacturing such a surface acoustic wave device, at least one interdigital transducer (referred to as "IDT" hereinafter) is formed on a piezoelectric substrate. In surface acoustic wave devices, the number of IDTs, and the shape and number of other electrodes are appropriately selected according to the desired component.
When used as a filter or a resonator, the surface acoustic wave device must be manufactured to achieve necessary frequency characteristics. However, there is a problem that frequency characteristics of surface acoustic wave devices deviate from the designed characteristics due to variations of physical properties between various piezoelectric substrates and poor manufacturing precision in forming IDTs.
Therefore, a method of manufacturing a surface acoustic wave device has been proposed in which a frequency can be adjusted by controlling the thickness of a polyimide resin coating layer formed on the surface acoustic wave device (for example, see Japanese Laid-Open Patent Publication Nos. 61-208916 and 8-32392).
More specifically, Japanese Laid-Open Patent Publication No. 61-208916 discloses that an operation frequency of a surface acoustic wave device can be adjusted by depositing a polyimide resin coating layer on a surface acoustic wave substrate so as to have a predetermined thickness.
Japanese Laid-Open Patent Publication No. 8-32392 discloses a method in which a fluorinated polyimide resin coating is formed, by a spin coating method to have a thickness corresponding to an amount of required frequency adjustment, on the substrate surface where a surface acoustic wave propagates. Japanese Laid-Open Patent Publication No. 8-32392 further discloses that the fluorinated polyimide resin coating may be etched by a plasma containing oxygen gas for further adjustment of the frequency.
However, the above-explained conventional methods using a polyimide resin coating can only decrease the operation frequency of the surface acoustic wave device. That is, it has been impossible to adjust an operation frequency of a surface acoustic wave device so that the operation frequency is increased.
In addition, when the fluorinated polyimide resin coating is etched by using a plasma containing oxygen gas, it is necessary to use a vacuum apparatus. Thus, each surface acoustic wave device requiring frequency adjustment must be loaded into a vacuum chamber and the vacuum chamber must be evacuated thereafter. As a result, it takes considerable time to adjust the frequency of a large number of surface acoustic wave devices, which makes the frequency adjustment process have a low through-put.